468 grim. CERAMICS AND CLAY COMPOSITION [Ch. 25 



excellent working properties, and considerably less clay mineral may 

 be present if the product is to be formed by dry pressing. Unsorted 

 non-clay mineral particles provide greater density in the formed ware 

 than particles of uniform size and are, therefore, most desirable. 



Small amounts of organic material enhance the plastic properties of 

 some clays. However, not all types of organic material increase the 

 plasticity, and it is not possible to predict generally the influence of 

 organic material in a particular clay. The plastic properties of a clay 

 vary somewhat with the content and character of exchangeable bases 

 and soluble salts. Thus, the addition of soda ash (Na 2 C0 3 ) to a clay 

 carrying exchangeable calcium tends to reduce the amount of water 

 necessary to obtain a given degree of plasticity. 



Suspension Characteristics 



All the clay minerals form suspensions in water if their particle 

 size is sufficiently small and if there is the proper electrolyte con- 

 tent. The montmorillonite minerals, attapulgite, and perhaps the 

 sepiolite-like minerals are more easily placed in suspension than the 

 other clay minerals, because they break down very easily in water to 

 exceedingly small particles, because of their adsorptive power for elec- 

 trolytes, and because of their influence on the state of the water im- 

 mediately surrounding them (Grim, 1942). 



When certain montmorillonite clays carry sodium as the exchange- 

 able ion, their suspensions in water have a high degree of thixotropy. 

 The thixotropic property would be detrimental in some ceramic ap- 

 plications, as in slip casting, and advantageous in others, as in steel 

 enameling. Small amounts of montmorillonite in a clay or added to 

 a clay greatly enhance suspension-forming characteristics. 



Bonding Strength 



As shown in Table 4, the montmorillonite clay minerals have greater 

 bonding strength than any of the other clay minerals studied so far. 

 Montmorillonite clay carrying calcium (Mississippi) has higher green 

 strength than a similar clay carrying sodium (Wyoming). Sodium 

 montmorillonite clays have the highest dry strength. Green strength 

 refers to the strength of a compacted test piece containing tempering 

 water. Dry strength is the strength of the compacted test piece after 

 drying at 110° C. for 2 hours. The bonding strength of illite and 

 kaolinite clays increases as the particle size decreases, and halloysite 

 clays in an intermediate state of hydration have higher strength than 

 either the high- or low-hydration form. 



